The 'sulc' conveys information on how far removed a particular vertex point on a surface is from a hypothetical "mid-surface" that exists between the gyri and sulci. This surface is chosen so that the "mean" of all these displacements is zero. The 'sulc' gives a indication then of linear distance and displacements, i.e. how "deep" and how "high" are brain folds
Thickness of the outer gray matter cortical ribbon of the brain